Partially crystallizing lead-free enamel composition for automobile glass

ABSTRACT

A ceramic enamel composition contains 40-80% by weight of at least one lead-free metal oxide frit containing precursors of Zn 2  SiO 4 , such as ZnO and SiO 2 , wherein the amount of Zn 2  SiO 4  precursors is less than about 35% by weight of the at least one lead-free metal oxide frit, 0.05-15% by weight of a zinc silicate seed material, and 19-37% by weight of a pigment. The zinc silicate seed material preferably contains crystalline Zn 2  SiO 4 . The enamel preferably also contains a bismuth silicate seed material. The ceramic enamel can be employed as a coating around the periphery of automotive glass and is effective in improving appearance and reducing degradation of underlying adhesives by ultraviolet radiation.

This application is a division of application Ser. No. 08/784,924, filedJan. 16, 1997 now U.S. Pat. No. 5,783,507 issued Jul. 21, 1998.

FIELD OF THE INVENTION

The present invention relates to ceramic enamel compositions for use inautomotive windshields, sidelights and backlights.

BACKGROUND OF THE INVENTION

Ceramic enamel compositions can be used for a variety of applications,such as decorative coatings for glassware, chinaware, and the like. Theyare especially useful in forming colored borders around glass sheetsused for automotive windshields, sidelights and backlights. The coloredborders enhance appearance as well as prevent degradation of underlyingadhesives by UV radiation.

In general, these enamel compositions consist mainly of a glass frit, acolorant and an organic vehicle. They are applied to a desired region ofthe substrate and subsequently fired to burn off the organic vehicle andfuse the ceramic solids to the surface of the substrate.

Glass sheets for automotive use are generally coated in the desiredregion with the ceramic enamel composition and then subjected to apressure forming process at elevated temperatures. During this treatmentthe enamel melts and fuses to the glass substrate and the glass isformed into a desired final shape. However, many previous coatingsexhibit a tendency to adhere to the materials covering the forming die,e.g., a fiberglass or metal fiber covered die, because theseconventional enamels have a low viscosity after melting and tend tostick to other materials at high temperature. Accordingly, such previousenamels are not suitable for use in glass forming processes in which theheated glass coated with enamel is pressure formed with a die.

Various approaches have been suggested in order to facilitate theforming of glass sheets with a ceramic enamel coated thereon without theenamel adhering to the forming die. For example, U.S. Pat. Nos.4,596,590 and 4,770,685 (issued to Boaz) propose the addition of a lowvalent metal oxide powder, e.g., cuprous oxide, to the paint compositionto provide a non-stick barrier between the coating and thefiberglass-covered forming die. U.S. Pat. Nos. 4,684,389; 4,857,096;5,037,783 and EP 490,611 (issued to Boaz), propose adding finely dividedzinc metal powder for a similar effect. The use of an iron metal powderis proposed in U.S. Pat. No. 4,983,196 (issued to Stotka).

A purportedly improved anti-stick ceramic enamel composition is proposedby U.S. Pat. Nos. 5,153,150; 5,208,191 and 5,286,270 (issued to Rudereret al.) wherein a seed powder containing Zn₂ SiO₄ is combined with theglass frit portion of the composition. The glass frit portion comprisesat least 35 percent by weight precursors for crystalline Zn₂ SiO₄, moreparticularly, at least 30 weight percent ZnO and at least 5 weightpercent SiO₂.

A further shortcoming of a number of previous ceramic enamel systems isthat they employ a lead-containing glass frit. For environmentalconsiderations it is desirable to avoid the use of any lead-containingsystem.

Along these lines, U.S. Pat. No. 4,882,301 (issued to Gettys et al.)proposes use of a crystallizing amount of Cd₂ SiO₄ with a leadborosilicate glass. This reference states that Zn can be substituteddirectly for Cd in the glass formulation; however, U.S. Pat. No.5,208,191 indicates that when zinc is substituted for cadmium, theresults have been less than desirable.

Also, while several of the above-mentioned enamel systems may performsatisfactorily in conventional glass forming processes, some may not besuitable for use in the newly-developed "deep bend" processes forforming automotive glass. Moreover, the enamel compositions must resistcertain chemical agents which they may contact.

The previous enamel compositions suffer from one or more of thedeficiencies noted above. In contrast, the present invention provides aceramic enamel composition that avoids these shortcomings.

SUMMARY OF THE INVENTION

The present invention is for a lead-free ceramic enamel composition thatforms an at least partially crystallizing zinc silicate material on aglass substrate upon fusing at high temperature.

A ceramic enamel composition of the present invention comprises 40-80%by weight of at least one lead-free metal oxide glass frit whichcontains precursors of Zn₂ SiO₄, e.g., ZnO and SiO₂, of 0.05-15% byweight of a zinc silicate seed material, and 20-35% by weight of a blackpigment. The sum of the weight percentage amounts of the aforementionedprecursors of Zn₂ SiO₄ provided by one or more frits is less than about35%, and more than about 15% by weight of the frit or frits.

The zinc silicate seed material of an instant composition is preferablyprovided as seed crystals in the composition, which upon firing providenuclei for further crystal growth. It is preferred that at least aportion of the zinc silicate seed material is crystalline in nature.Preferably, the crystalline seed material comprises at least about 95%by weight of crystalline Zn₂ SiO₄, for instance, approximately 100% byweight of crystalline Zn₂ SiO₄.

Another aspect of the invention involves including an additionalcrystalline material, such as a bismuth silicate seed material, in anenamel composition of the invention. A bismuth silicate seed material ispreferably provided in at least about 1% by weight of the enamelcomposition. Preferably, the bismuth silicate seed material comprises acrystalline bismuth silicate, such as crystalline Bi₁₂ SiO₂₀, Bi₄(SiO₄)₃, Bi₂ SiO₅, and mixtures thereof, in an amount of about 2 toabout 7% by weight.

A method of preparing an instant ceramic enamel composition comprisescombining in no particular order the aforementioned components in thedesired amounts, and optionally combining them with an organic vehicle.

The present invention also contemplates a method of using theaforementioned ceramic enamel with a glass substrate, e.g., to form acolored border and/or UV resistant border around its periphery. Thus, aglass substrate is provided with a ceramic enamel coating by applying anaforementioned ceramic enamel composition to the glass substrate,optionally applying pressure to the coated glass substrate, and firingthe substrate to fuse components of the ceramic enamel composition tothe substrate. Among the advantages of an instant enamel are itsexcellent anti-stick properties, good consistency, broad temperaturefiring range, low stress, and low cost.

The invention will now be described in more detail with reference toexamples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a ceramic enamel composition thatcontains the following components: (1) at least one conventionallead-free oxide glass frit containing precursors of Zn₂ SiO₄ ; (2) azinc silicate seed material; (3) a colorant (pigment); and, optionally,(4) a vehicle for components (1)-(3). The zinc silicate seed material isbelieved to assist in nucleating and growing microcrystallinestructures, e.g., crystalline zinc silicate phases, in the compositionupon firing. Although vehicle (4) is preferably provided in thecomposition, it can be omitted, and applied later, e.g., at the time ofsilkscreening, without departing from the essence of the presentinvention.

Typically, an instant ceramic enamel composition comprises 40-80% byweight of one or more of the aforementioned lead-free oxide glass frits,0.05-15% by weight of a zinc silicate seed material, and 20-35% byweight of a pigment. More preferably, the lead-free oxide frit isprovided in the composition in an amount of 60-65% by weight, the zincsilicate seed material is provided in an amount of 1-5% by weight, andthe pigment is provided in an amount of 25-33% by weight. The sum of theweight percentage amounts of the precursors of Zn₂ SiO₄ provided by oneor more of the aforementioned frits is less than about 35% by weight ofthe frit or frits, and the total of weight percentages of theabove-mentioned lead-free metal oxide frit(s), zinc silicate seedmaterial, and pigment does not exceed 100%.

Preferably, an instant ceramic enamel composition comprises 60-63% byweight of a lead-free metal oxide frit or frits containing precursors ofZn₂ SiO₄, 1-3% by weight of a zinc silicate seed material, and 27-30% byweight of a black pigment. It is also preferred that the total of theweight percentages for the precursors of Zn₂ SiO₄ provided by one ormore frits is less than about 35% by weight, but greater than about 20%by weight.

As for the ratio of Zn₂ SiO₄ precursors in the glass frit or frits,preferably, the amount of zinc oxide and silicon dioxide provided in anaforementioned at least one lead-free frit is 5-20% and 20-30%,respectively, whenever the total amount of Zn₂ SiO₄ precursors is 35% byweight. More preferably, the amount of zinc oxide provided in the fritor frits is in the range of 10% to 15%, and the amount of silicondioxide provided in the frit or frits is in the range of 20% to 25%,whenever the total amount of Zn₂ SiO₄ precursors is less than about 35%by weight. The zinc precursor can be provided solely by one frit and thesilicon precursor can be provided by a different frit, as long as Zn₂SiO₄ is formed whenever the enamel composition is fused.

By "precursors of Zn₂ SiO₄ " is meant substances that when fired at hightemperature react to form Zn₂ SiO₄. Chief among such precursors are ZnOand SiO₂. Other precursors can be used equivalently to these substances,and are readily apparent to those skilled in the art. Such otherprecursors include polymers, e.g., siloxanes, and discrete compounds,e.g., organometallic compounds, which decompose to form ZnO or SiO₂ uponfiring at elevated temperature. The ZnO and SiO₂, or parent compounds,can be provided either in the same lead-free frit, or in differentfrits, which upon firing fuse to generate Zn₂ SiO₄.

The zinc silicate seed material can be selected from any known phase ofthe Zn/Si phase system; however, zinc orthosilicate (Zn₂ SiO₄) ispreferred. Preferably, the zinc silicate seed material comprises atleast about 90% by weight of crystalline Zn₂ SiO₄. More preferably, thezinc silicate seed material comprises at least about 95% and up to 100%by weight of crystalline Zn₂ SiO₄.

As referred to herein, the terms crystal, crystalline, microcrystalline,and the like, mean that the subject material is sufficiently crystalline(ordered) to reveal one or more discrete phase by X-ray diffractiontechniques.

While not wishing to be bound by theory, it is believed that thepresence of the zinc silicate seed material causes nucleation and growthof crystals leading to increased refractoriness and devitrification. Thedevitrification involves the separation of microcrystalline structures,such as Zn₂ SiO₄, and the like, in the fused enamel. The presence ofthese microcrystalline structures in the fused enamel is believed togreatly reduce the tendency of the enamel to stick to surfaces, e.g.,pressing pads, during the shaping of the glass substrate at elevatedtemperature.

In respect to a lead-free oxide frit employed in the invention, aconventional ceramic oxide frit, such as a zinc-bismuth based frit, canbe employed. The frit can contain a boron source in addition to a zincsource. For instance, a frit composed of zinc oxide and boron oxide, andoptionally additional materials, can be used. Also, a frit composed ofzinc borosilicate, or one composed of a noncrystalline zinc silicatematerial, can be used. Preferably, such a frit is formulated to generatein situ upon heating the requisite zinc silicate microcrystallinestructures. In practice it is preferred to include a crystalline zincsilicate seed material directly in the enamel composition. At least someoxide frit is desirable in the composition in order to provide a flux.

A crystalline zinc silicate material suitable for use in the presentinvention can be prepared according to any of a number of well-knownmethods. For instance, Zn₂ SiO₄ (CAS Registry No. 13597-65-4) can beprepared by heating zinc oxide (ZnO) and SiO₂ in a molar ratio of 2:1 at1300° C. for 72 hours. Other methods of preparing these and relatedmaterials are readily apparent to the skilled practitioner.

The particle size for an instant zinc silicate seed material ispreferably in the range of 1 to 4 microns, more preferably about 1.8microns.

Typically, it is preferred also to include a bismuth silicate seedmaterial in an instant composition. While not wishing to be bound bytheory, it is believed that the presence of the bismuth silicate seedmaterial causes nucleation and growth of crystals leading to increasedrefractoriness and devitrification. Devitrification involves theseparation of microcrystalline structures, such as Bi₁₂ SiO₂₀, Bi₂(SiO₃)₄, and the like, in the fused enamel. The presence of thesemicrocrystalline structures in the fused enamel is believed to helpreduce the tendency of the enamel to stick to surfaces during theshaping of the glass substrate at elevated temperature.

Preferred bismuth silicate seed materials for this type of reactivesystem can include, but are not limited by, the compounds Bi₁₂ SiO₂₀,Bi₄ (SiO₄)₃, Bi₂ SiO₅, and mixtures thereof. Any one or all of thesecompounds are preferably crystalline and may be present as a mixturewithin the same crystalline material.

A crystalline bismuth silicate material suitable for use in the presentinvention can be prepared according to any of a number of well-knownmethods. For instance, Bi₁₂ SiO₂₀ (CAS Registry No. 12377-72-9) can beprepared by heating bismuth oxide and silicon dioxide in a 6:1 molarratio at up to 840° C. for 16 hours National Bureau of Standards,Monogr., 25:22 (1985)!. Bi₄ (SiO₄)₃ (CAS Registry No. 15983-20-7) can beprepared by firing a 2:3 ratio of bismuth oxide and silica at 780° C.for 50 hours, regrinding, and firing at 830° C. for 18 hours Roob, etal., North Dakota State Univ., JCPDS Grant-in-Aid Report (1980)!. Bi₂SiO₅ (CAS Registry No. 12027-75-7) can be prepared by melting a 1:1ratio of bismuth oxide and silicon dioxide at 1000-1040° C., quenchingin water, and crystallizing at 400-520° C. for one week Keller, et al.,Mineralogisch-Petro-graphisches Institut, Univ. Heidelberg, Germany,JCPDS Grant-in-Aid Report (1984)!. Other methods of preparing these andrelated materials are readily apparent to the skilled practitioner.

The particle size for an instant bismuth silicate seed material ispreferably in the range of 1 to 4 microns, more preferably about 1.8microns.

Additional crystalline materials can be incorporated in the formulationas fillers, such as alumina-silicate compounds, calcium silicatecompounds, boro-alumina-silicate compounds, soda-calcia-alumina-silicatecompounds, feldspar compounds, titania, zinc borate, and mixturesthereof. Still further metallic and/or oxide materials, e.g., iron,silicon, zinc, and the like, can be added to enhance desired properties,such as resistance to silver bleed-through, especially when theirpresence promotes the nucleation and growth of the requisite zincsilicate and bismuth silicate microcrystalline structures.

As presently preferred, an enamel composition of the invention containsa base glass frit which is at least one conventional lead-free frit,such as those commercially available from Cerdec Corporation(Washington, Pa.). Such frits can be employed alone or can be blended toachieve the desired properties. Other suitable zinc-containing frits arewell-known in the art.

A representative formulation for a suitable lead-free frit of thepresent invention has a composition as shown in Table I:

                  TABLE I    ______________________________________    Oxide       Weight % Range    ______________________________________    ZnO          3-15    SiO.sub.2   10-25    Bi.sub.2 O.sub.3                20-55    B.sub.2 O.sub.3                 2-20    Na.sub.2 O   1-10    K.sub.2 O   0-3    Li.sub.2 O  0-3    CaO          0-10    SrO          0-10    TiO.sub.2   0-5    Al.sub.2 O.sub.3                0-5    ZrO.sub.2   0-5    F.sub.2     0-3    ______________________________________

A method of making a frit of this type is disclosed in U.S. Pat. No.5,346,651 (issued to Oprosky et al.). Such frits have a sufficiently lowfiring temperature to ensure adequate adhesion to the substrate and alsopossess low density characteristics.

Exemplary zinc-containing frits suitable for use with the invention arecommercially available from Cerdec Corporation as E-8018, E-8009, andE-8008.

A pigment of a ceramic enamel of the invention can be any of thosecommercially available. Particularly preferred pigments are commerciallyavailable from Cerdec Corporation as *2991 pigment, which is a copperchromite black pigment, *2980 pigment, which is a cobalt chromium ironblack pigment, and *2987 pigment, which is a nickel manganese ironchromium black pigment.

A vehicle to be employed for use with an instant composition is selectedon the basis of its end use application. The vehicle should adequatelysuspend the particulates and burn off completely upon firing of thecomposition on the substrate. Vehicles are typically organic and includecompositions based on pine oils, vegetable oils, mineral oils, lowmolecular weight petroleum fractions, tridecyl alcohol, synthetic andnatural resins, and the like.

Correspondingly, UV-base vehicles are equally applicable for use in theinvention. Such UV-base vehicles are well-known in the art and aregenerally composed of polymerizable monomers and/or oligomerscontaining, for example, acrylate or methacrylate functional groups,together with photoinitiators and polymerization inhibitors.Representative vehicles are disclosed in U.S. Pat. Nos. 4,306,012 and4,649,062. As is recognized, such vehicles are cured with ultravioletradiation after application to the substrate.

The specific vehicle and amounts employed are selected based upon thespecific components of the composition and the desired viscosity. Ingeneral, the amount of the vehicle is about 10 to about 40% by weightbased upon the total weight of the solid enamel composition.

In general, the enamel compositions are viscous in nature, with theviscosity depending upon the application method employed and the enduse. For purposes of screen-printing, viscosities ranging from 10,000 to80,000, and preferably 35,000 to 65,000, centipoises at 20° C., asdetermined on a Brookfield Viscometer, #7 spindle at 20 rpm, areappropriate.

To prepare an enamel composition of the invention, a frit is ground to afine powder using conventional methods and is combined in any order withan aforementioned zinc silicate seed material, a pigment, any bismuthsilicate seed material, and any fillers. When the zinc silicate seedmaterial of the composition is desired to comprise a crystalline zincsilicate, it is also added. Other oxides, as discussed above, can beadded, as well as materials which resist silver bleed-through. More thanone representative of each of the different types of componentsmentioned above can be provided in the enamel composition.

Once the enamel composition is prepared it can be applied to a glasssubstrate in a conventional manner, such as by screen printing, decalapplication, spraying, brushing, roller coating, and the like. Screenprinting is preferred when the composition is applied to glasssubstrates.

After application of the composition to a substrate in a desiredpattern, the applied coating is then fired to bond the enamel to thesubstrate. The firing temperature is generally determined by the fritmaturing temperature, and preferably is in a broad temperature range.Typically, the firing range for an instant composition is in the rangeof 1100-1350° F., more preferably in the range of 1200-1300° F., andmost preferably about 1250° F. Whenever pressure is applied to thesubstrate, the pressure is typically in the range of 1 to 3 psi,preferably about 2 psi.

The following examples represent preferred embodiments of the invention.They are presented to explain the invention in more detail, and do notlimit the invention.

EXAMPLES

Several exemplary enamel compositions were prepared by conventionalmethods using the components listed in Table II. The indicated weightpercentages are calculated based on a printing medium being excludedfrom the composition. Frit E-8018 referred to in Table II is a lead-freebismuth-borosilicate frit commercially available from CerdecCorporation, which has a ZnO content of 14.4 and an SiO₂ content of20.1% by weight. Pigment *2991 is a black pigment commercially availablefrom Cerdec Corporation. Zinc silicate seed was prepared by reactingzinc oxide and silicon dioxide as described, with X-ray diffractionanalysis indicating the presence of zinc orthosilicate. Bismuth silicateseed was prepared by reacting bismuth trioxide and silicon as describedwith X-ray diffraction analysis indicating the presence of eulytite.Identical materials were used for each component in the examples.

                  TABLE II    ______________________________________    Component      Ex. 1  Ex. 2     Ex. 3                                         Ex. 4    ______________________________________    E-8018         63.64  63.64     63.64                                         62.00    *2991 pigment  30.30  30.30     30.30                                         26.00    Bismuth silicate seed                   5.00   4.00      3.03 5.00    Zinc silicate seed                   1.06   2.06      3.03 7.00    ______________________________________

The present invention has been described above by way of illustrationwith reference to examples. However, it should be appreciated that theinvention is not limited to the particular embodiments set forth aboveand that certain obvious modifications can be practiced within the scopeof the appended claims.

What is claimed is:
 1. A method of forming a glass substrate with anadherent ceramic enamel coating which comprisesapplying to a glasssubstrate a ceramic enamel composition comprising: 40-80% by weight ofat least one lead-free metal oxide glass frit containing precursors ofZn₂ SiO₄, wherein the sum of the weight percentage amounts of saidprecursors of Zn₂ SiO₄ in said lead-free metal oxide frit is less thanabout 35% by weight of said lead-free metal oxide frit; 0.05-15% byweight of a zinc silicate seed material; and 19-37% by weight of apigment, wherein the sum of the weight percentage amounts of said atleast one lead-free metal oxide frit, zinc silicate seed material, andpigment does not exceed 100%, and heating the coated glass substrate toan elevated temperature to fuse components of the ceramic enamelcomposition to the glass substrate.
 2. The method of claim 1, furthercomprising subjecting the heated glass to a forming pressure with a die,and separating the formed glass from the die.